Synthesis of photoluminescent carbon dots for the detection of cobalt ions

We have developed a simple assay for the sensing of cobalt ions (Co2+), based on the analyte induced photoluminescence (PL) quenching of carbon dots (C-dots). The C-dots (mean diameter 3.6 +/- 0.3 nm) prepared from L-cysteine through a simple hydrothermal process at 300 degrees C for 2 h have a quantum yield of 13.2%. The C-dots have strong blue PL with a maximum PL intensity at 395 nm under an excitation wavelength of 325 nm. Through the reactions of Co2+ ions with cysteine molecules/residues on the surfaces of the C-dots, non-photoluminescent CoxSy nanoparticles are formed. As-formed CoxSy nanoparticles and the C-dots further form aggregates in the solution, leading to PL quenching. The C-dot probe allows detection of Co2+ ions over a concentration range from 10 nM to 100 mu M (R-2 = 0.992). This reliable, rapid, sensitive, and selective C-dot probe has been utilized for the determination of the concentrations of Co2+ ions in vitamin B-12 and natural water samples.